In keeping with a theme of landscape definitions, I want to introduce the topic of walls. Nearly all of the projects we construct include some type of wall, most often a retaining wall. It is the Bay Area, after all- most homeowners have to contend with some amount of slope in their landscape, and creating useable outdoor spaces often requires retaining walls to flatten sloped areas. There are a lot of nuances to designing and building walls, so I’m going to break wall construction down over the next few blog posts so that you can throw around wall-related terms like ‘surcharge’ and ‘footing’ with the pros.
Retaining walls are used to retain soil- as a tool for leveling ground and preventing soil erosion. They can also serve as built in seating and to delineate spaces. People often refer to the building of retaining walls as ‘terracing’. There are many types of retaining walls, which I’ll be going through in later posts. First, some basics about retaining walls:
‘Curbs’ or ‘Edging’: These are very short retaining walls- less than 12” in height. We use these often when grading (leveling) spaces with a minimal slope, or when building stairs into a hillside. Along stairs, edging is often referred to as ‘cheekwall.’ As with the Moss Rock edging in the photo below, the edging keeps the stairs free of debris which could wash onto them when it rains.
‘Seatwalls’: We use the term seatwall all the time, although it’s not a technical term. The meaning is pretty obvious though- these walls are 14” – 18” tall, roughly the height your chair is from the ground to the seat. This is a wall that can be comfortably perched on, and which usually requires a ‘cap’ (a flat piece of stone or a wood plank) on a wall of reasonable seat height. This is a great opportunity to use the walls you need for soil retention as built-in seating, eliminating the need for furniture in smaller areas. Cushions can be added to make seatwalls even more comfortable, give them a pop of color or reflect your personal style. In the photo below, a seatwall was built directly in front of a taller retaining wall, creating a back to the seat. A retaining wall of 3’ or more appears even higher when it is uphill from where you’re standing, so seatwalls can also break up that vertical element- lessening that Machu Picchu effect.
Short Retaining Walls: Retaining walls less than 3’ in vertical height. Typically, this is the maximum height a wall can be without requiring a permit or engineering (unless you’re lucky enough to live in Piedmont, where 2’ – 6” is your maximum allowable height. And I’m betting on that changing in the future, so build your retaining walls while you can, Piedmonters!!!) In truth, simple wall engineering becomes less feasible when heights of 3’ are exceeded. The pressure of the earth behind the wall becomes too great for simple wall solutions, so we need to call in the big guns. By big guns, I’m referring to both materials (Steel I-Beams, for example) and people (namely, engineers). In the photo below, a series of short retaining walls was installed flanking new stairs on a steep slope. This is an example of terracing a site.
Tall (Engineered) Retaining Walls: Walls over 3’ in vertical height. These walls are for significant slope retention or to affect dramatic grade change. We’ve installed these walls when carving out a driveway for a house far upslope from the road, to help eliminate damage to house foundations from significant water runoff, and even to keep an eroding road from falling into a client’s front yard. These walls require calculations and drawings from an engineer, and permits from your municipality. This usually means a significant investment- some tall retaining walls cost upwards of $500 per linear foot. If you can safely avoid building walls of this height, you will save yourself a pretty penny. Sometimes it seems like the most straightforward option when trying to maximize space, but because of aesthetics as well as cost, we install them rarely.
Footings: All the walls described above are load-bearing walls, even the curbs and edgings. So they all must have a footing. This is the portion of the wall that sits below grade (grade being the level of the ground adjacent to the wall). Think of it like you would the foundation of your house. You don’t see it, but it’s doing a very important job. The footing distributes the weight of the retaining wall structure to the soil below, supporting the weight of the wall and the soil behind it. Often when you see failing retaining walls, it’s because a footing wasn’t included or wasn’t properly installed. Piers, which are vertical pillars that support walls, are a type of footing. For tall engineered walls, piers can be 20’ deep or more, which is a significant part of the cost of building them.
Surcharge: Surcharge is the added weight above a wall. So if the area above your retaining wall isn’t flat, but continues to slope up, it has a surcharge. Patios, pools, driveways, or other weighty residential landscape elements are also surcharges. In some local municipalities, like the City of Berkeley, surcharge on a retaining wall requires engineering and/or permits, even if the desired wall height is less than 3’.
Drainage: Drainage is extremely important in relation to retaining walls. I’ll get more into why as I explore different types of retaining walls, but poor drainage can compromise pretty much any retaining wall, no matter how strong, over time. Water is powerful stuff, people, and it has to be considered.
Soil Conditions: This is another integral factor in retaining wall design. We have varied soil conditions in the Bay Area. Many of you in San Francisco proper have lots of sand in your landscape, which drains quickly, but doesn’t compact well, which can affect soil stability. On the opposite end of the spectrum, East Bay residents often have an abundance of clay in the soil. Ever tried to plant something in August in Oakland? It’s no picnic. In fact I’d rather have rocky soil than clay. Clay soils expand and contract dramatically based on rain, which can cause movement in built walls.
Using best practices in retaining wall design and construction, which consider soil conditions, drainage, surcharge, and local code in addition to style and personal aesthetics, is imperative. Choosing the best type of wall for your site takes all of these factors into account. Next time, I’ll get into some of the different types of retaining walls we can build, and the pros and cons of using them in your landscape.